C++ PATCH for c++/89906 (GCC 8 backport)

[Marek Polacek]

In order to fix 89906 in GCC 8, we need to backport 86098.
I think the patch is safe to be backported.

Tested x86_64-linux, ok for 8?

2018-06-12  Jason Merrill  <ja...@redhat.com>

        PR c++/86098 - ICE with template placeholder for TTP.
        * typeck.c (structural_comptypes) [TEMPLATE_TYPE_PARM]: Check
        CLASS_PLACEHOLDER_TEMPLATE.

--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -1375,6 +1375,11 @@ structural_comptypes (tree t1, tree t2, int strict)
         template parameters set, they can't be equal.  */
       if (!comp_template_parms_position (t1, t2))
        return false;
+      /* If T1 and T2 don't represent the same class template deduction,
+         they aren't equal.  */
+      if (CLASS_PLACEHOLDER_TEMPLATE (t1)
+         != CLASS_PLACEHOLDER_TEMPLATE (t2))
+       return false;
       /* Constrained 'auto's are distinct from parms that don't have the same
         constraints.  */
       if (!equivalent_placeholder_constraints (t1, t2))
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/class-deduction58.C
@@ -0,0 +1,16 @@
+// PR c++/86098
+// { dg-additional-options -std=c++17 }
+
+template <class _Res> class future;
+template <class T> T&& declval();
+
+template<template <class...> class T>
+struct construct_deduced {
+  template <class... AN>
+  using deduced_t = decltype(T{declval<AN>()...});
+  template<class... AN>
+  deduced_t<AN...> operator()(AN&&... an) const;
+};
+
+template<class T>
+future<T> future_from(T singleSender);